The present invention relates to a method of measuring a distance between radiocommunication devices, and a device suitable for implementing such a method.
It is often useful to know the position of a radiocommunication device. Such is the case, in particular, when contextual information, that is, the relevance of which depends on the position of the user of the equipment, needs to be transmitted. Such is also the case when a mobile radiocommunication device is linked to a meshed network or to an “ad hoc” network which uses a routing algorithm based on the position of the mobile device. The term “ad hoc” network is used to mean a transmission network capable of recognizing changes to the latter autonomously, that is, with no outside intervention. Radio networks deployed to determine the position of mobile devices having individual electronic labels, or for guiding a holder of a mobile device in a determined site are other examples of applications to which the invention can be applied.
Several methods already exist which make it possible to determine the position of a radiocommunication device. Among these, the triangulation-based methods require a map of reference radio stations whose positions are listed to be stored. Other methods are based on measuring the receive strength of a radio signal, but they require a calibrated radio transmission strength scale to be implemented. Such methods consequently require storage, measurement and/or control means that are complex.
Also known is a method of estimating a distance between two radiocommunication devices by determining a propagation time for radio signals transmitted between said devices. Such a time is called time of flight, and is determined as follows:                a first device transmits a radio signal requesting a distance measurement and simultaneously triggers a chronometer internal to that device;        a second device receives the request signal and, after an intermediate time known to both devices, sends an acknowledgement radio signal to the first device; and        the first device stops the chronometer when it receives the acknowledgement signal.        
The distance between the two devices is then estimated by subtracting the intermediate delay from the clocked time, and by dividing the remaining time obtained by two times the propagation speed of the radio signals. Such a method of estimating distance is relatively exact when the two devices are far apart from each other, and when the radio signals are propagated in a straight line between them. However, when the two devices are fairly close together, the accuracy of a duly obtained distance estimation is strongly affected by an uncertainty relating to the intermediate delay between the reception of the request by the second device and the transmission of the acknowledgement signal by said device.
Furthermore, when there are several propagation paths between the two devices, that is, in space diversity cases, the estimated distance does not necessarily correspond to the distance between the two devices which is measured in a straight line. It most often corresponds to the propagation path followed by a main part of the energy of the transmitted radio signals. When a large part of the energy of the radio signals is subject to at least one reflection between the two devices, the result of the distance estimation that is obtained can be very much greater than the real value of the distance between the two devices, which is measured in a straight line.
To obtain an estimation of the straight-line distance that separates two devices, channel measurement frames are used for each of the request and acknowledgement signals that are transmitted to determine the time of flight of the radio signals. Such channel measurement frames are well known. Their structure makes it possible to determine differences between the times of flight of radio signals that follow separate propagation paths. They also make it possible to detect the shortest of the propagation paths followed from the transmitting device by a part of the energy of the radio signal. The distance is then estimated according to the time of flight of the request and acknowledgement signals which corresponds to the shortest propagation path between the two devices. For most propagation medium configurations, the result obtained corresponds to the measurement of the distance in a straight line. All the subsequent events that occur within each device for the relevant communication are then identified relative to the shortest propagation time between the two radio devices.
Now, a channel measurement frame is particularly long, compared to a communication frame. Consequently, the distance measuring method based on an exchange of channel measurement frames as described above presents the following drawbacks:                two channel measurement frames are transmitted in total, which corresponds to a large quantity of energy consumed in the radiocommunication devices. Such energy consumption is disadvantageous in the case of mobile devices with autonomous energy supply;        each channel measurement frame requires a significant time to construct and transmit said frame. If the device for which such a frame is intended is not available, the energy consumed in the transmitting device and the radio resource used to transmit the frame are lost;        the intermediate delay between the reception of the request frame and the transmission of the acknowledgement frame must be greater than the time to construct the channel measurement frame which is used for the acknowledgement signal. It is therefore long, which generates an imprecision in the estimation of the distance between the two devices when the internal clock of at least one of the two devices is liable to drift. Such an imprecision can generate a major error in the estimation of the time of flight for two radio devices close to each other;        to enable the time of flight of the frames to be measured, the terminal receiving the request frame unilaterally mobilizes the radio resource on expiry of the intermediate delay to transmit the acknowledgement frame. Such pre-emption of the radio resource needs to occur after said intermediate delay, with a maximum delay of the order of a nanosecond, only. For this, other communications involving the device transmitting the acknowledgement frame must be interrupted if necessary. The result is a disruption that is all the greater when the acknowledgement frame is long;        finally, the communication is continued according to the radio signals received by each device that follow the shortest propagation path. These signals can correspond to a far weaker reception energy than that of the signals transmitted via another propagation path. The communication then has a dependability level lower than that which would result from the use of radio signals received with a greater energy.        
It would be possible to overcome this last drawback by having two receivers within each radio device. The first receiver could be synchronized on the received radio signals that correspond to the shortest propagation path, and the second receiver could be synchronized on the radio signals that present the greatest energy on reception. However, the device would then be complex and would have a high energy consumption.